Fulweiler (U.S. Pat. No. 3,583,897) refers to the use of a rotating cylindrical cathode and indicates that it is desirable to keep the voltage as low as possible and to maintain a current density of from about 8 to 10 amperes per square foot of cathode surface. Cave (U.S. Pat. No. 3,925,184) recognized that the production of sulfide ions is a problem which must be controlled through control of plating current. DeSante (U.S. Pat. No. 3,183,177) considered the reversal of current in order to strip material from a plate on which it had been deposited. Duisenberg (U.S. Pat. No. 2,791,555) indicated that silver ions can be extracted from used photographic or "hypo" solutions via a plurality of disc-shaped cathodes which are negatively biased relative to a plurality of anode elements. Adams (U.S. Pat. No. 3,342,718) rotated the cathode and regulated the power supply. Tolle (U.S. Pat. No. 4,049,512) also suggested a motor-rotated cathode. Crellin (U.S. Pat. No. 3,642,594) recovered silver and regenerated used photographic fixing solutions electrochemically using high-current densities; he refers to electroplating silver from a used fixing solution containing 300 parts of hypo, 10 parts of "acedic acid", approximately 4 parts of silver and 1,000 parts of water.
Anderson (U.S. Pat. No. 3,715,299) recognized that continuous disturbance of the boundary layer surrounding the cathode structure significantly improves the electroplating process, while discouraging the formation of deleterious by-products. He indicates that his continuous circulation virtually eliminates any tendency towards sulfiding even with materailly-higher current densities. Geyken (U.S. Pat. No. 4,081,816) neutralizes excess quantities of developer by adding simple neutralizing agents, e.g., acetic acid, apparently to obtain chemical neutrality. Although Cooper (U.S. Pat. No. 3,663,416) employs a low voltage, it appears that he actually does obtain sulfiding.
Willier (U.S. Pat. No. 2,615,839) and Mandroian (U.S. Pat. No. 3,072,557) refer to voltages of from 1 to 1.2 volts. Snow (U.S. Pat. No. 3,477,926) refers to preventing the local drop in silver-ion concentration by agitation.
Lindau (U.S. Pat. No. 3,510,413) refers to treating the bath within a plating tank with about 6.4 ounces of glacial acetic acid. Graham (U.S. Pat. No. 3,577,334) prefers to operate at a constant voltage and indicates that the optimum is ordinarily in the range of from 0.5 to 1 volt.
Scheidegger (U.S. Pat. No. 4,139,431) refers to controlling pH, but does not disclose the specific voltage employed; he adds exhausted developer before or during electrolysis to control pH.